1. Field of the Invention
The present invention relates to a brittle workpiece splitting system for splitting a substrate made of a hard, brittle material by locally heating the substrate to cause a crack in the substrate. More particularly, the present invention relates to a brittle workpiece splitting system capable of quickly achieving a splitting process for cutting off an edge part of a substrate in a high processing quality, and to a brittle workpiece splitting method to be carried out by the brittle workpiece splitting system. This patent application is based on application Nos. 2004-296215 and 2005-249928 the content of which are incorporated hereinto by reference.
2. Description of the Related Art
A conventional splitting method of splitting a planar substrate, such as a glass substrate or the like for a liquid crystal display panel or a plasma display panel, locally heats the substrate and cools the locally heated substrate. A crack is caused in the substrate by the agency of thermal stress (tensile stress) induced in the locally heated and locally cooled substrate and the substrate splits.
The conventional splitting method irradiates a part of a substrate mounted on a stage with a laser beam to locally heat the substrate so that a crack is caused in the substrate by the agency of a thermal stress (tensile stress) induced in the heated part. A heating point on which the laser beam falls to locally heat the substrate is moved along an intended split line to extend the crack in the substrate along the intended split line. A coolant is sprayed on the locally heated part to locally cool the substrate. Consequently, the thermal stress (tensile stress) induced in the locally cooled part can be increased and the growth of the crack can be promoted.
In this conventional splitting method, it is necessary to make the crack extend accurately in the substrate along the intended split line in order to accurately split the substrate.
In related art splitting methods proposed in JP-A Nos. 7-328781, 7-323384 and 2003-34545, controlled are irradiation conditions for irradiating a substrate with a laser beam and the positional relation between an irradiation point at which a laser beam falls and a cooling point to make a crack grow stably in the substrate.
The foregoing related art splitting methods need an expensive control system and an optical system to carry out complicated control operations for controlling irradiation with the laser beam and spraying the coolant on the substrate. Consequently, the prior art splitting methods need an expensive, complex apparatus.
A related art splitting method, other than the foregoing splitting methods, using an improved substrate holding mechanism to cause a crack in the substrate and to make the crack extend has been proposed. A related art splitting method disclosed in JP-A No. 2002-110589 causes a crack to extend linearly along an intended split line by holding parts of a substrate respectively on the opposite sides of the in tended split line by fixing jigs to rigidify the substrate so that the parts of the substrate are evenly rigidified. Abutting parts, which come into contact with the substrate, of the fixing jigs are made of rubber, namely, an elastic material having comparatively low elasticity.
The splitting method mentioned in JP-A No. 2002-110589 is able to make a crack extend accurately linearly in the substrate along the split line when the split line extends in a central part of the substrate. However, a problem that a crack extends along a meandering line arises when this related art splitting method is applied to a trimming process for cutting off an edge part of a substrate and the splitting method is unable to cause a crack to extend accurately linearly. This problem becomes more remarkable when the intended split line is close to an edge, namely, a free end, of the substrate and is at a short distance of, for example, about 10 mm from the edge. The term “accurate linear extension of the crack” concerns an order of (±) several tenth millimeters or below. The extension of the crack with a deviation in the range of about ±0.1 to about ±0.2 mm from the intended split line is accurate extension of the crack. The extension of the crack with a deviation in the range of about ±0.4 to about ±1 mm from the intended split line is inaccurate extension of the crack. More preferably, the deviation of the crack from the intended split line is about (±) several tens micrometers on the opposite sides of the intended split line. Refer to “JSME International Journal Series A, Vol. 68, No. 670, pp. 93” for further information about the accuracy of linearity.